Why are the sun and moon the same size in the sky?

The “diamond ring” effect is seen toward the end of a total solar eclipse, as the Moon just begins to reveal the Sun’s bright surface (photosphere) at the end of totality. This eclipse was photographed from Bolivia on 3 November 1994

(Image: Reverend Ronald Royer / SPL)

A partial solar eclipse seen from Stonehenge, UK on 31 May 2003

(Image: Les Wilson / Rex Features)

A total solar eclipse, as seen from Towan Beach, Newquay, Cornwall, UK on 11 August 1999

(Image: Sam Morgan Moore / Rex)

The different stages of a solar eclipse, as seen in 2001 from Sambia, Russia

(Image: Martin Rietze / WestEnd61 / Rex)

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It is one of the most glorious pieces of natural theatre. Assuming you spend your life on the same part of the Earth’s surface, you might witness it once – if you are particularly lucky or very long-lived, perhaps twice. But a total solar eclipse is worth the wait. At the height of totality, the fit of sun and moon is so perfect that beads of sunlight can only penetrate to us through the rugged valleys on the lunar surface, creating the stunning “diamond ring” effect.

It is all thanks to a striking coincidence. The sun is about 400 times as wide as the moon, but it is also 400 times further away. The two therefore look the same size in the sky – a unique situation among our solar …